Could be holy popish thinking (10)
I believe the answer is:
philosophy
'thinking' is the definition.
(thesaurus)
'could be holy popish' is the wordplay.
'could be' indicates anagramming the letters.
'holy'+'popish'='holypopish'
'holypopish' anagrammed gives 'PHILOSOPHY'.
(Other definitions for philosophy that I've seen before include "Pursuit of truth and understanding" , "Set of principles derived from deep thought?" , "First P of PPE" , "Platonic affair" , "Calmness of temper" .)